1. Field of the Invention
The present invention relates to a read only memory (ROM) with memory cells each capable of storing one of three states.
2. Description of the Related Arts
To increase the degree of integration of an integrated circuit, attention has been paid to a memory cell which is called a multi-value levelsmemory cell. (See, for example, U.S. Pat. Nos. 4,449,203, 4,503,518, and IEEE Journal of Solid State Circuits, Vol. SC-16, No. 5, Pages 522-529). The storing of data into such a memory cell having multi- value levels is generally carried out by changing a mutual- conductance (transconductance) gm of the memory cell transistor. For example, gm of a memory cell transistor can be selected from three kinds of gm, i.e., gm(1), gm(2), and gm(3), which correspond to three kinds of states: state-1, state-2, and state-3, respectively.
In the prior art, the discrimination of these three kinds of gm is carried out by providing a first reference transistor having a gm which is at an intermediate value between state-1 and state-2, and a second reference transistor having a gm which is at an intermediate value between state-2 and state-3, and comparing the output from the reference transistors with the output from the memory cells. In this case, problems arise in that it is difficult to further enhance the precision in manufacturing of a reference transistor, and the circuit construction of the ROM device is relatively complicated. Accordingly, the degree of integration of the ROM device cannot be easily increased.